Sheet feeder and image forming apparatus

ABSTRACT

There is provided a sheet feeder including: a lifting unit that lifts a loading portion having a recording medium loaded thereon; a position detecting unit that detects whether the uppermost surface of the recording medium is disposed at a predetermined first position; a sheet feed mechanism that feeds the recording medium from the loading portion; a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; and a control unit that, when the position detecting unit detects that the uppermost surface of the recording medium is disposed at the first position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a second position that is higher than the first position by a distance determined based on cumulative value of the thickness of the recording medium detected by the thickness detecting unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 USC 119 from Japanese Patent Application No. 2008-293377 filed on Nov. 17, 2008.

BACKGROUND

1. Technical Field

The present invention relates to a sheet feeder and an image forming apparatus.

2. Related Art

In the related art, an image forming apparatus, such as a copier and a printer, includes a sheet feeder that feeds a recording medium, on which images are recorded, to an image forming unit.

The recording medium which is loaded on a bottom plate that can be lifted up by a wire is stored in a tray of the sheet feeder.

In the image forming apparatus, when the uppermost recording medium in the tray is sequentially fed to the image forming unit, the number of recording medium loaded in the tray is reduced, and the position of the uppermost recording medium becomes lower than the lower limit of the range in which the recording medium can be fed to the image forming unit, the bottom plate is lifted up such that the uppermost recording medium is adjusted to be within the sheet feedable range.

SUMMARY

An aspect of the invention provides a sheet feeder including:

a lifting unit that lifts a loading portion having at least one recording medium loaded thereon;

a position detecting unit that detects whether the uppermost surface of the recording medium loaded on the loading portion is disposed at a predetermined first position;

a sheet feed mechanism that feeds the recording medium from the loading portion;

a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; and

a control unit that, when the position detecting unit detects that the uppermost surface of the recording medium is disposed at the first position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a second position that is higher than the first position by a distance determined based on cumulative value of the thickness of the recording medium detected by the thickness detecting unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating the structure of an image forming system according to a first exemplary embodiment;

FIG. 2 is a diagram schematically illustrating the structure of a sheet feeder according to the first exemplary embodiment;

FIG. 3 is a block diagram illustrating the structure of the image forming system according to the first exemplary embodiment;

FIG. 4 is a flowchart illustrating the flow of a sheet feed process according to the first exemplary embodiment;

FIG. 5 is a flowchart illustrating the flow of an initial operation of the sheet feeder according to the first exemplary embodiment;

FIG. 6 is a diagram schematically illustrating a sheet feed operation according to the first exemplary embodiment;

FIG. 7 is a flowchart illustrating the flow of a sheet feed process according to a second exemplary embodiment;

FIG. 8 is a flowchart illustrating the flow of an initial operation of a sheet feeder according to the second exemplary embodiment;

FIG. 9 is a diagram schematically illustrating a sheet feed operation according to the second exemplary embodiment;

FIG. 10 is a diagram schematically illustrating the structure of an image forming system according to a third exemplary embodiment;

FIG. 11 is a block diagram illustrating the structure of the image forming system according to the third exemplary embodiment;

FIG. 12 is a flowchart illustrating the flow of a sheet feed process according to the third exemplary embodiment;

FIG. 13 is a diagram schematically illustrating the structure of a sheet feeder according to a modification; and

FIG. 14 is a flowchart illustrating the flow of a sheet feed process according to a modification of the third exemplary embodiment.

DETAILED DESCRIPTION

Hereinafter exemplary embodiments of the present invention will be described in detail.

First Exemplary Embodiment

As shown in FIG. 1, an image forming system 10 according to a first exemplary embodiment includes a sheet feeder 12 and an image forming apparatus 16.

The sheet feeder 12 feeds a recording medium 14 to the image forming apparatus 16.

For example, a sheet can be used as the recording medium 14. However, the recording medium 14 is not limited to the sheet. Any medium may be used as long as the image forming apparatus 16 can form an image thereon.

The image forming apparatus 16 includes an image reading unit 18 and an image forming apparatus main body 20.

The image reading unit 18 includes, for example, a line CCD sensor and a scanning mechanism that scans the recording medium with the line CCD sensor. The image reading unit 18 reads the image of a set document in response to, for example, instructions from a user.

The image forming apparatus main body 20 includes an image forming unit 22. The image forming unit 22 forms an image on the recording medium 14 fed from the sheet feeder 12.

The image forming unit 22 includes a photoconductor drum 24, a charging unit 26, an exposure device 28, a developing unit 30, a transfer roller 32, and a fixing unit 34.

The charging unit 26, the exposure device 28, the developing unit 30, and the transfer roller 32 are arranged around the photoconductor drum 24.

The photoconductor drum 24 is a cylindrical photoconductor, and is rotationally driven at a predetermined rotation speed.

The charging unit 26 uniformly charges the surface of the photoconductor drum 24.

The exposure device 28 exposes the photoconductor drum 24 according to image data of an image to be formed on the recording medium 14 and forms an electrostatic latent image on the photoconductor drum 24. The exposure device 28 may include an LED print head obtained by arranging, for example, plural LEDs. In addition, the exposure device 28 may be an optical scanning device that includes a semiconductor laser, a collimator lens, a polygon mirror, and an fθ lens, and performs scanning exposure based on image data on the photoconductor drum 24.

The developing unit 30 develops the electrostatic latent image formed on the photoconductor drum 24 with toner.

The transfer roller 32 transfers the toner image on the photoconductor drum 24 onto the recording medium 14. The transferred toner image is fixed when the recording medium 14 passes between a pressure roller 36 and a heating roller 38 provided in the fixing unit 34.

The recording medium 14 is discharged to a discharge tray 44 through a sheet outlet 42 provided in the side surface of the image forming apparatus 16 by a discharge roller 40.

Next, the structure of the sheet feeder 12 will be described with reference to FIG. 2.

The sheet feeder 12 includes a tray 46. A lifting portion on which the recording medium 14 is loaded, for example, a bottom plate 48 is provided in the tray 46.

One end of a wire 50 is connected to the bottom plate 48 while being suspended by a pulley 52. In addition, the other end of the wire 50 is connected to a driving gear 56 that is rotationally driven by a lift-up motor 54. When the driving gear 56 is rotated in the direction of an arrow A, the wire 50 suspended by the pulley 52 is wound to lift up the bottom plate 48. When the driving gear 56 is rotated in the direction of an arrow B, the wire 50 suspended by the pulley 52 is unwound to drop the bottom plate 48.

The sheet feeder 12 includes a sheet feed mechanism that feeds the recording medium 14. The sheet feed mechanism includes a sheet feed unit having, for example, a sheet transport roller 58, a sheet transport roller rotating shaft 60, and a sheet transport roller supporting arm 62, a position detecting unit, such as a sheet feedable position detecting sensor 64, a feed roller 66, a retard roller 68, a thickness detecting sensor 70, a takeaway pinch roller 76, and a takeaway drive roller 78.

The thickness detecting unit, for example, the thickness detecting sensor 70 is formed by mounting a thickness detecting actuator 74 to a rotary encoder 72.

In the sheet feeder 12, when the recording medium 14 is in the range in which the sheet can be fed, the sheet transport roller 58 is rotated in the direction of an arrow C, with the sheet transport roller 58 contacted with the uppermost surface of the recording medium 14 loaded on the bottom plate 48, to feed the recording medium 14.

The sheet transport roller supporting arm 62 that supports the vertical movement of the sheet transport roller 58 is mounted to the sheet transport roller 58.

The sheet transport roller supporting arm 62 rotates about a sheet transport roller supporting arm rotating shaft 65. The sheet transport roller supporting arm 62 is rotated upward when a sheet transport roller solenoid 87 (not shown) is turned on. The sheet transport roller supporting arm 62 is rotated downward by the elastic force of a spring (not shown) when the sheet transport roller solenoid 87 is turned off. In this way, the sheet transport roller 58 is moved in the vertical direction represented by an arrow D.

The sheet feedable position detecting sensor 64 is arranged close to the sheet transport roller supporting arm 62. The sheet feedable position detecting sensor 64 detects the position of the uppermost surface of the recording medium 14 loaded on the bottom plate 48.

The sheet feedable position detecting sensor 64 detects the uppermost surface of the recording medium 14, without being affected by the vertical movement of the sheet transport roller 58.

The fed recording medium 14 are separated one by one by the feed roller 66 and the retard roller 68 that is rotationally driven in a direction opposite to the rotation direction of the feed roller 66 by a torque limiter (not shown).

The separated recording medium 14 is transported to the image forming apparatus 16 by the takeaway pinch roller (driven roller) 76 and the takeaway drive roller 78.

The thickness detecting sensor 70 is arranged between the feed roller 66 and the takeaway pinch roller 76.

The thickness detecting sensor 70 is arranged at a position where the thickness detecting actuator 74 is lifted up whenever the recording medium 14 is fed.

When the recording medium 14 is fed and the thickness detecting actuator 74 is lifted up by a distance corresponding to the thickness of the recording medium 14, the rotary encoder 72 is rotated at an angle corresponding to the amount of lift. The rotary encoder 72 outputs a signal corresponding to the rotation angle. That is, the rotary encoder 72 outputs a signal corresponding to the thickness of the recording medium 14.

Next, the structure of the main components of an electrical system of the sheet feeder 12 will be described with reference to FIG. 3.

As shown in FIG. 3, the sheet feeder 12 includes a control unit 80, the thickness detecting sensor 70, a thickness cumulative value calculating unit 82, a sheet feedable position detecting sensor 64, a sheet transport roller motor 84, a feed roller motor 86, a retard roller motor 88, a takeaway drive roller motor 90, a lift-up motor 54, a sheet transport roller solenoid 87, and an I/F (interface) 92.

Whenever the recording medium 14 is fed, the value of the thickness of the recording medium 14 detected by the thickness detecting sensor 70 is input to the thickness cumulative value calculating unit 82, and the input thickness value is accumulated (added). Then, the thickness cumulative value calculating unit 82 outputs the calculated cumulative value to the control unit 80.

The image forming apparatus 16 transmits an instruction signal that instructs the sheet feeder 12 to feed the recording medium 14 to the I/F 92. The instruction signal includes, for example, information about the number of sheets to be fed. The I/F 92 outputs the instruction signal from the image forming apparatus 16 to the control unit 80.

When receiving the instruction signal for feeding the recording medium 14 from the image forming apparatus 16 through the I/F 92, the control unit 80 controls the operation of the sheet transport roller motor 84 to feed the recording medium 14 to the image forming apparatus 16.

Next, a sheet feed process routine performed by the control unit 80 of the sheet feeder 12 provided in the image forming system 10 according to this exemplary embodiment will be described with reference to FIG. 4.

When the user operates the image forming apparatus 16 to select an image forming service such as copy, the routine is performed.

First, in Step 100, an initial operation processing routine shown in FIG. 5 is performed. In the initial operation processing routine, as shown in FIG. 5, in Step 120, it is determined whether the sheet feedable position detecting sensor 64 is turned on. The sheet feedable position detecting sensor 64 is turned on when the uppermost surface of the recording medium 14 is in a sheet feedable range 94 shown in FIG. 6.

The sheet feedable range 94 is defined by, for example, a first position A, which is the lower limit of the range in which the medium 14 can be fed, and a predetermined second position B, which is the upper limit of the range in which the recording medium 14 can be fed and is higher than the first position A.

When the uppermost surface of the recording medium 14 is lower than the lower limit of the range in which the recording medium 14 can be fed, it is difficult to feed the recording medium. When the uppermost surface of the recording medium 14 is higher than the upper limit of the range in which the recording medium 14 can be fed, it is difficult to feed the recording medium. The first position A is not limited to the lower limit of the range, and any position may be used as the first position as long as it is in the range in which the recording medium 14 can be fed.

In addition, the second position B is not limited to the upper limit of the range, and any position may be used as the second position as long as it is higher than the first position and is in the range in which the recording medium 14 can be fed. That is, the sheet feedable range 94 is not limited to between the lower limit and the upper limit, and any range may be used as long as the recording medium can be fed therewithin.

The sheet feedable position detecting sensor 64 is turned off when the uppermost surface of the recording medium 14 is lower than the first position A. Therefore, when the sheet feedable position detecting sensor 64 is turned on, the current position of the uppermost surface of the recording medium 14 is in the range in which the recording medium can be fed. Therefore, it is not necessary to lift up the bottom plate 48, and the process proceeds to Step 102 in FIG. 4.

If it is determined in Step 120 that the sheet feedable position detecting sensor is turned off, the process proceeds to Step 122. In this state, the position of the uppermost surface of the recording medium 14 is not within the sheet feedable range. That is, since the position of the uppermost surface of the recording medium 14 is lower than the first position A, the lift-up motor 54 is driven to lift up the bottom plate 48 until the sheet feedable position detecting sensor 64 is turned on. Then, the uppermost surface of the recording medium 14 is lifted up to the first position A in the sheet feedable range 94.

Then, the process proceeds to Step 124 to drive the lift-up motor 54 such that the bottom plate 48 is further lifted up by a predetermined threshold value. The threshold value corresponds to the sheet feedable range 94 in FIG. 6. That is, the threshold value corresponds to a thickness cumulative value when the recording medium 14 is fed up to the first position A that is lower than the second position B, which is the upper limit of the sheet feedable range. Therefore, the bottom plate 48 is lifted up such that the uppermost surface of the recording medium 14 is lifted up to the second position B in the sheet feedable range 94. In this way, the recording medium 14 is lifted up to the second position B.

Then, in Step 126, the control unit 80 outputs an instruction signal to the cumulative value calculating unit 82 to reset the thickness cumulative value calculated by the cumulative value calculating unit 82. Then, the thickness cumulative value calculating unit 82 resets the thickness cumulative value.

The position of the uppermost surface of the recording medium 14 is within the sheet feedable range by the initial operation processing routine.

Then, the recording medium is fed in Step 102 in FIG. 4. That is, the sheet transport roller motor 84 is controlled such that the recording medium 14 loaded on the bottom plate 48 is fed to the image forming apparatus 16.

In Step 104, it is determined whether the sheet feedable position detecting sensor 64 is turned off, that is, whether the uppermost surface of the recording medium 14 is lower than the first position A.

If it is determined in Step 104 that the sheet feedable position detecting sensor 64 is not turned off, that is, if the uppermost surface of the recording medium 14 is within the sheet feedable range 94, the process proceeds to Step 110.

If it is determined in Step 104 that the sheet feedable position detecting sensor 64 is turned off, that is, sheet feeding is performed and the position of the uppermost surface of the recording medium 14 is lower than the first position A, i.e., beyond the sheet feedable range, the process proceeds to Step 106. In Step 106, the control unit receives the calculated thickness cumulative value output from the cumulative thickness value calculating unit 82.

Then, in Step 108, the lift-up motor 54 is driven such that the bottom plate 48 is lifted up by the thickness cumulative value input to the control unit 80. In this way, the uppermost surface of the recording medium 14 is lifted up to the second position B.

Then, in Step 109, the control unit 80 outputs an instruction signal to the thickness cumulative value calculating unit 82 to reset the thickness cumulative value calculated by the thickness cumulative value calculating unit 82. Then, the thickness cumulative value calculating unit 82 resets the thickness cumulative value.

Then, in Step 110, it is determined whether the number of sheets corresponding to the instruction signal from the image forming apparatus 16 is completely fed.

If it is determined in Step 110 that the number of sheets corresponding to the instruction signal is not completely fed, the process returns to Step 102, and the same operation as described above is repeatedly performed.

Second Exemplary Embodiment

Next, a second exemplary embodiment will be described.

In this exemplary embodiment, the same components as those in the first exemplary embodiment are denoted by the same reference numerals, and a description thereof will be omitted.

The structures of an image forming system 10, a sheet feeder 12, and an image forming apparatus 16 are the same as those according to the first exemplary embodiment, and a description thereof will be omitted.

The structure of the main components of an electrical system according to the second exemplary embodiment is the same as that according to the first exemplary embodiment, and a description thereof will be omitted.

However, in this exemplary embodiment, a sheet feedable position detecting sensor 64 detects that the uppermost surface of the recording medium 14 is disposed at the first position A. That is, the sheet feedable position detecting sensor 64 is turned on when the recording medium 14 is disposed at the first position A, and is turned off when the position of the recording medium 14 is lower than the first position A.

The first position A is not limited to the upper limit of the sheet feedable range, and any position may be used as the first position as long as it is in the range in which the recording medium 14 can be fed. In addition, the second position B is not limited to the lower limit of the sheet feedable range, and any position may be used as the second position as long as it is lower than the first position and is in the range in which the recording medium 14 can be fed. That is, the first and second positions may be in the range in which the recording medium can be fed, similar to the first exemplary embodiment.

A sheet feed process routine performed by the control unit 80 of the sheet feeder 12 provided in the image forming system 10 according to this exemplary embodiment will be described with reference to FIG. 7.

Similar to the first exemplary embodiment, when the user operates the image forming apparatus 16 to select an image forming service such as copy, the routine is performed.

First, in Step 150, an initial operation processing routine shown in FIG. 8 is performed. A thickness cumulative value may be forcibly reset, for example, when power is supplied to the image forming apparatus at the beginning, when the main power supply is turned on or off, and when the recording medium 14 is added to the tray 46. In this case, even when the uppermost surface of the recording medium 14 is not actually disposed at the second position B, which is the upper limit of the sheet feedable range, the thickness cumulative value is reset. Therefore, first, in Step 200, it is determined whether the thickness cumulative value calculated by the thickness cumulative value calculating unit 82 is reset.

If it is determined in Step 200 that the thickness cumulative value is not reset, the process proceeds to Step 202. On the other hand, if it is determined that the thickness cumulative value is reset, the process proceeds to Step 204.

In Step 202, it is determined whether the thickness cumulative value calculated by the thickness cumulative value calculating unit 82 is a predetermined threshold value or more.

The threshold value corresponds to the sheet feedable range 94 shown in FIG. 9. That is, the threshold value corresponds to the thickness cumulative value when the recording medium 14 is fed up to the second position B that is lower than the first position A, which is the upper limit of the sheet feedable range.

If the determination result in Step 202 is “YES”, the process proceeds to Step 204.

In Step 204, the lift-up motor 54 is driven to lift up the bottom plate 48 until the sheet feedable position detecting sensor 64 is turned on. Then, the uppermost surface of the recording medium 14 is lifted up to the first position A.

Further, the process proceeds to Step 206, the control unit 80 outputs an instruction signal to the thickness cumulative value calculating unit 82 to reset the thickness cumulative value calculated by the thickness cumulative value calculating unit 82. Then, the thickness cumulative value calculating unit 82 resets the thickness cumulative value.

On the other hand, if the determination result in Step 202 is “NO”, the current position of the position of the uppermost surface of the recording medium 14 is within the sheet feedable range. Therefore, it is not necessary to lift up the bottom plate 48, and the process proceeds to Step 152 in FIG. 7.

The position of the uppermost surface of the recording medium 14 is within the sheet feedable range by the initial operation processing routine.

Then, the recording medium is fed in Step 152 in FIG. 7. That is, the sheet transport roller motor 84 is controlled such that the recording medium 14 loaded on the bottom plate 48 is fed to the image forming apparatus 16.

In Step 154, the control unit receives the calculated thickness cumulative value output from the cumulative value calculating unit 82.

Then, in Step 156, it is determined whether the thickness cumulative value received in Step 154 is a predetermined threshold value or more. That is, it is determined whether the position of the uppermost surface of the recording medium 14 is lower than the second position B.

If it is determined in Step 156 that the thickness cumulative value is less than the threshold value, i.e., if the uppermost surface of the recording medium 14 is within the sheet feedable range 94, the process proceeds to Step 162.

If it is determined in Step 156 that the thickness cumulative value is the threshold value or more, i.e., if sheet feeding is performed and the position of the uppermost surface of the recording medium 14 is lower than the second position B, that is, beyond the sheet feedable range, the process proceeds to Step 158. In Step 158, the lift-up motor 54 is driven to lift up the bottom plate 48 until the sheet feedable position detecting sensor 64 is turned on. In this way, the uppermost surface of the recording medium 14 is lifted up to the first position A.

Then, the process proceeds to Step 160, the control unit 80 outputs an instruction signal to the thickness cumulative value calculating unit 82 to reset the thickness cumulative value calculated by the thickness cumulative value calculating unit 82. Then, the thickness cumulative value calculating unit 82 resets the thickness cumulative value.

Next, the process proceeds to Step 162, it is determined whether the number of sheets corresponding to the instruction signal from the image forming apparatus 16 is completely fed.

If it is determined in Step 162 that the number of sheets corresponding to the instruction signal is not completely fed, the process returns to Step 152, and the same operation as described above is repeatedly performed.

Third Exemplary Embodiment

Next, a third exemplary embodiment will be described with reference to FIG. 10.

In this exemplary embodiment, the same components as those in the first and second exemplary embodiments are denoted by the same reference numerals, and a description thereof will be omitted.

An image forming system 10A includes a sheet feeder 13, an image forming apparatus 16, and an image reading unit 18.

As shown in FIG. 10, the sheet feeder 13 includes plural sheet feed units. For example, the sheet feeder 13 includes three sheet feed units 13A-13C in this exemplary embodiment.

The sheet feeder 13 is provided in an image forming apparatus main body 20.

The sheet feed units 13A to 13C have the same structure as the sheet feeder 12 shown in FIG. 2, and characters “A” to “C” are added to ends of the reference numerals of the components having the same structure.

The image forming apparatus 16 and the image reading unit 18 are the same as those in the first exemplary embodiment, and thus a description thereof will be omitted.

The sheet feed units 13A to 13C feed recording medium 14A to 14C to the image forming unit 22, respectively.

The recording medium 14A to 14C are loaded in trays 46A to 46C provided in the sheet feed units 13A to 13C, respectively.

For example, the recording medium 14A to 14C may be different types of recording medium having different thicknesses.

In the following description, when the sheet feed units 13A to 13C are not individually specified, they are simply referred to as a “sheet feed unit 13”. Similarly, when the recording medium 14A to 14C are not individually specified, they are simply referred to as a “recording medium 14”.

Next, the structure of the main components of an electrical system of the sheet feed unit 13 will be described with reference to FIG. 11.

The same components as those according to the first exemplary embodiment are denoted by the same reference numerals, and a description thereof will be omitted.

In the third exemplary embodiment, sheet feedable position detecting sensors 64A to 64C, thickness detecting sensors 70A to 70C, thickness cumulative value calculating units 82A to 82C, lift-up motors 54A to 54C, sheet transport motors 84A to 84C, feed roller motors 86A to 86C, retard roller motors 88A to 88C, takeaway drive motors 90A to 90C, and sheet transport roller solenoids 87A to 87C are provided so as to correspond to the sheet feed units 13A to 13C, respectively.

In the third exemplary embodiment, a storage unit 81 is provided.

The storage unit 81 stores predetermined threshold values corresponding to the recording medium 14A and 14B respectively loaded in the sheet feed units 13A and 13B.

Next, a sheet feed process routine performed by the control unit 80 of the image forming system 10A according to this exemplary embodiment will be described with reference to FIG. 12.

When the user operates the image forming apparatus 16 to select an image forming service such as copy, the routine is performed.

First, in Step 250, among the threshold values stored in the storage unit 81, a threshold value corresponding to the recording medium 14 designated by the selected image forming service is read.

In Steps 252 to 256, the same process as that in Steps 150 to 156 according to the second exemplary embodiment is performed on the sheet feed unit 13 having the designated recording medium 14 loaded therein.

However, in Step 258, the thickness cumulative value of the sheet feed unit 13 corresponding to the designated one of the recording medium 14A to 14C is read from any one of the thickness cumulative value calculating units 82A to 82C.

In Steps 260 to 264, the same process as that in Steps 158 to 162 according to the second exemplary embodiment is performed on the sheet feed unit 13 having the designated recording medium 14 loaded therein.

Next, a sheet feed process routine performed by the control unit 80 of an image forming system 10A according to a modification of the third exemplary embodiment will be described with reference to FIG. 14.

In Step 250, predetermined threshold values corresponding to the sheet feed units 13A to 13C are read from the storage unit 81. In Steps 252 to 256, the same process as that in Steps 150 to 156 according to the second exemplary embodiment is performed on the sheet feed unit 13 having the designated recording medium 14 loaded therein. However, in Step 258, a thickness cumulative value corresponding to the designated one of the sheet feed units 13A to 13C is read from any one of the thickness cumulative value calculating units 82A to 82C. In Steps 260 to 264, the same process as that in Steps 158 to 162 according to the second exemplary embodiment is performed on the sheet feed unit 13 having the designated recording medium 14 loaded therein.

In the first to third exemplary embodiments, it is described the thickness detecting sensor 70 includes the rotary encoder 72 and the thickness detecting actuator 74, however, the invention is not limited thereto. For example, as shown in FIG. 13, the thickness detecting sensor 70 may include a laser displacement meter 96 and a roller 98. In this case, the laser displacement meter 96 measures the amount of movement of the roller 98 in the vertical direction to detect the thickness of the fed recording medium 14. In the first to third exemplary embodiments, the sensor is provided so as to detect the position of the uppermost surface of the recording medium 14 loaded on the bottom plate 48 without being affected by the movement of the sheet transport roller 58 in the vertical direction. However, a sheet feedable position detecting sensor may be provided so as to detect the position of the sheet transport roller supporting arm 62 in the vertical direction, and the sheet feedable position detecting sensor may detect the position of the sheet transport roller supporting arm 62 when the sheet transport roller 58 comes into contact with the uppermost surface of the recording medium 14 loaded on the bottom plate 48, thereby detecting the position of the uppermost surface of the recording medium 14 loaded on the bottom plate 48.

The structures of the sheet feeders, the image forming systems, and the image forming apparatuses according to the exemplary embodiments are just examples, and various modifications and changes of the invention can be made without departing from the concept of the invention.

In the above-described exemplary embodiments of the invention, the image forming unit 22 of the image forming apparatus 16 forms images by electrophotography. However, the image forming unit may form images using other methods such as an ink jet recording method.

In addition, the flow of the processing program according to each of the exemplary embodiments is just an example, and various modifications and changes of the process flow can be made without departing from the concept of the invention. For example, the process order of the steps may be changed, unnecessary steps may be removed, and a new step may be added. 

1. A sheet feeder comprising: a lifting unit that lifts a loading portion having at least one recording medium loaded thereon; a position detecting unit that detects whether the uppermost surface of the recording medium loaded on the loading portion is disposed at a predetermined first position; a sheet feed mechanism that feeds the recording medium from the loading portion; a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; and a control unit that, when the position detecting unit detects that the uppermost surface of the recording medium is disposed at the first position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a second position that is higher than the first position by a distance determined based on cumulative value of the thickness of the recording medium detected by the thickness detecting unit.
 2. A sheet feeder comprising: a lifting unit that lifts a loading portion having at least one recording medium loaded thereon; a position detecting unit that detects whether the uppermost surface of the recording medium loaded on the loading portion is disposed at a predetermined first position; a sheet feed mechanism that feeds the recording medium from the loading portion; a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; a thickness cumulative value calculating unit that calculates the cumulative value of thickness of the at least one recording medium detected by the thickness detecting unit; and a control unit that, when the cumulative value calculated by the thickness cumulative value calculating unit is equal to a predetermined threshold value indicating that the uppermost surface of the recording medium loaded on the loading portion is disposed at a second position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a predetermined first position higher than the second position.
 3. The sheet feeder of claim 2, wherein: there are a plurality of sheet feed units comprising the sheet feed mechanism and the lifting unit; the sheet feeder further comprises a storage unit that stores the threshold value relating to the position of the recording medium of each of the sheet feed units, and the control unit reads the threshold value for the recording medium fed by any one of the plurality of sheet feed units from the storage unit and uses the read threshold value as the predetermined threshold value.
 4. The sheet feeder of claim 2, wherein: there are a plurality of sheet feed units comprising the sheet feed mechanism and the lifting unit; the sheet feeder further comprises a storage unit that stores respective threshold values of each of the sheet feed units, and the control unit reads a threshold value for the sheet feed mechanism of any one of the plurality of sheet feed units from the storage unit and uses the read threshold value as the predetermined threshold value.
 5. An image forming apparatus comprising: a sheet feeder; and an image forming unit that forms an image on at least one recording medium fed by the sheet feeder, wherein the sheet feeder includes: a lifting unit that lifts a loading portion having the recording medium loaded thereon; a position detecting unit that detects whether the uppermost surface of the recording medium loaded on the loading portion is disposed at a predetermined first position; a sheet feed mechanism that feeds the recording medium from the loading portion; a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; and a control unit that, when the position detecting unit detects that the uppermost surface of the recording medium is disposed at the first position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a second position that is higher than the first position by a distance determined based on cumulative value of the thickness of the recording medium detected by the thickness detecting unit.
 6. An image forming apparatus comprising: a sheet feeder; and an image forming unit that forms an image on at least one recording medium fed by the sheet feeder, wherein the sheet feeder includes: a lifting unit that lifts a loading portion having the recording medium loaded thereon; a position detecting unit that detects whether the uppermost surface of the recording medium loaded on the loading portion is disposed at a predetermined first position; a sheet feed mechanism that feeds the recording medium from the loading portion; a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; a thickness cumulative value calculating unit that calculates cumulative value of thickness of the at least one recording medium detected by the thickness detecting unit; and a control unit that, when the cumulative value calculated by the thickness cumulative value calculating unit is equal to a predetermined threshold value indicating that the uppermost surface of the recording medium loaded on the loading portion is disposed at a second position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a predetermined first position higher than the second position.
 7. The image forming apparatus of claim 6, wherein there are a plurality of sheet feed units comprising the sheet feed mechanism and the lifting unit; the sheet feeder further comprises a storage unit that stores the threshold value relating to the position of the recording medium of each of the sheet feed units, and the control unit reads the threshold value for the recording medium fed by any one of the plurality of sheet feed units from the storage unit and uses the read threshold value as the predetermined threshold value.
 8. The image forming apparatus of claim 6, wherein there are a plurality of sheet feed units comprising the sheet feed mechanism and the lifting unit; the sheet feeder further comprises a storage unit that stores respective threshold values of each of the sheet feed units, and the control unit reads a threshold value for the sheet feed mechanism of any one of the plurality of sheet feed units from the storage unit and uses the read threshold value as the predetermined threshold value. 